1. Field of the Invention
This invention relates to a novel propeller used for underwater purposes, in particular, a propeller having limp propeller blades or vanes.
2. Description of the Prior Art
Prior propellers are constructed of metal and, more recently, out of plastics or fiber reinforced composite plastics or "aerospace" material, and are structural in nature. Propellers made of conventional material are designed to be relatively stiff and light in order to support the applied loads. However, propellers made of such material are not particularly suitable acoustically when used for underwater purposes as they have a relatively low inherent or mechanical damping ability and relatively high surface flexural wave speeds, which translate to a higher acoustic sound radiation or propagation, i.e., the lower the inherent mechanical damping ability and the greater the surface flexural wave speed, the higher the acoustic radiation. Basically, decreasing the blade surface flexural wave speed will reduce the acoustic radiation, in the frequency range,
U.S. Pat. Nos. 4,097,193; 4,797,066; and 5,108,262 disclose improved propeller blades and the like. In particular, U.S. Pat. No. 4,097,193 discloses an elastomeric damping arrangement for damping vibrations of a structural member, such as a vibration prone airfoil, particularly a helicopter rotor blade which tends to vibrate under dynamic deformations. The damping materials disclosed are silicones, various rubber compounds or polyurethane.
U.S. Pat. No. 4,797,066 discloses a propeller blade that automatically optimizes its pitch to reduce cavitation thereof. The propeller blade is flexible and is comprised of plastic materials having anisotropic properties, i.e., materials with different physical characteristics in different directions. The propeller's improved properties are achieved by having the reinforcement applied in sections, with the reinforcing elements in one section extending in the same direction, but with the reinforcing elements of one section extending in direction other than the corresponding elements of another section.
U.S. Pat. No. 5,108,262 discloses a flexible high damping propeller comprising a unitary hub having an axis of rotation and multiple blades extending radially from the hub. Each blade comprises a tip portion made of a high density material having the shape that conforms to that of the propeller blade shape and an inner portion, which is contiguous and collinear with the tip portion, of multidimensionally braided fiber preforms of reinforcing fibers such as graphite, fiberglass, kevlar, polyethylene, etc., impregnated with a highly viscoelastic matrix material such as rubber, urethane, etc. Basically, when the propeller is in a loaded state (rotating), shear stresses occur in the viscoelastic material at the interface between the viscoelastic material and the relatively stiff fibers (reinforcing fibers). This high energy dissipating mechanism causes the majority of damping. For underwater use purposes, the composites have a density close to that of water which leads to increased radiation when compared with metal. Some vibrational frequencies propagate long distances in the water. To modify these frequencies, the natural frequency of the structure is changed by adding concentrated weight to the propeller blades to change the modal shape or response.
In U.S. Pat. No. 5,108,262, the propeller blades are designed to be limp under a static load (non-rotating condition). Once the blades rotate, however, they are subject to hydrodynamic forces that produce large longitudinal, circumferential, and tangential stresses on the propeller blades.